The following relates generally to position sensors and/or sensing, and more particularly, to an apparatus and method for sensing gear features, such as gear teeth and/or gear slots. The following also relates to magnetic effect sensing apparatuses including linear position sensing as well as the commonly known rotary position xe2x80x9cgeartooth sensorsxe2x80x9d that have a magnetically sensitive device for sensing ferrous objects or objects generally projecting from a rotating target and resembling the teeth of a gear. The apparatus and method is particularly useful for providing an efficient, portable, reliable, and extensible camshaft and/or crankshaft geartooth sensors.
Various sensors are known in the magnetic-effect sensing arts. Examples of common magnetic-effect sensors may include Hall effect and magnetoresistive technologies. Generally, these magnetic-effect sensors will respond to the change of magnetic field as influenced by the presence or absence of a ferromagnetic target object of a designed shape that passes through or by the sensory field of the magnetic-effect sensor.
When acting as a transducer, the magnetic-effect sensor generally outputs an electrical signal representative of the sensed magnetic field. The electrical signal can vary in amplitude and width so as to correspond to the shape, e.g., a profile, of the target object. The signal can be modified by various electronic circuitries for processing and conditioning so as to yield sensing and control information. The various electronics may be positioned either onboard or outboard of the magnetic-effect sensor""s packaging.
Such magnetic-effect sensing may be employed to detect gear features, such as gear teeth and/or gear slots. A magnetic-effect sensor deployed for this purpose is commonly referred to as a xe2x80x9cgeartoothxe2x80x9d sensor. Geartooth sensors may be used in the automotive arts to provide information to an engine controller for ignition timing control, fuel management, and other operations of the automotive power plant. For example, a geartooth sensor can be located in proximity to a ferrous target wheel positioned on a crankshaft of an engine to determine, for example, when the first piston is at top-dead center. Such determination may be made when the target wheel has features, e.g., teeth and slots that are properly keyed to mechanical operation of engine components.
As another example, a geartooth sensor can be located in proximity to a ferrous target wheel positioned on a camshaft of an engine to determine, for example, how to manage ignition timing. In one such embodiment, regularly spaced tooth-to-slot transitions yield a rhythmic, or regular, pulse pattern that can be used to determine the timing or xe2x80x9cclockingxe2x80x9d information necessary to run such functions of the engine as fuel injection and spark plug firing.
Further examples of magnetic-effect sensors can be found in of United States patents in the related art include: U.S. Pat. Nos. 6,404,168; 6,191,576; 6,232,832; 5,729,128; 5,694,040; 5,694,038; 5,650,719; 5,500,589; 5,497,084; 5,455,510; 5,414,355; 5,304,926 and 5,140,262. The entire content of each of these patents is incorporated herein by reference.
It is well known in the art that the waveforms produced by the magnetic-effect sensor change in response to varying xe2x80x9cair gapxe2x80x9d between the target and sensor faces. Also, differences among the biasing magnets used in the magnetic-effect sensor, temperature, mechanical stresses, irregular target feature spacing, etc., can vary the output of the magnetic-effect sensor. As a result, the point at which the magnetic-effect sensor changes state, i.e. the switch point, varies in time, or drifts, in relation to the degree of rotation of the target. The mechanical action of the engine as represented by the target, however, does not change. That is, there is a xe2x80x9ctrue pointxe2x80x9d on the target in angle, or degrees of rotation, related to a hard-edge transition. Consequently, there is a point at which the magnetic-effect sensor should change state to indicate a mechanical function of the engine.
But due to inherent limitations of the sensing system, the point at which the sensor changes state will vary by some amount from this true point. Unfortunately, the magnetic-effect sensor does not provide the proper tooth-to-slot (and slot-to-tooth) position accuracy, i.e., it is not really giving a timing signal accurately representing piston travel. Therefore, the system controlled by the sensor can be inefficient.
Several schemes are known in the art to reduce sensor inaccuracies by providing an adaptive threshold of waveform voltage at which to switch the magnetic-effect sensor. The adaptive threshold, which is used as reference for comparing the output of the magnetic-effect sensor, seeks to switch the sensor at a nearly constant angle in order to decrease switch point drift and increase accuracy of the sensor and efficiency of the engine.
Various known systems for producing an adaptive threshold include systems that set the adaptive threshold at a fixed level above a measured minimum magnetic bias signal and then compare the output of the magnetic-effect sensor to this fixed level. This function, however, does not convey information proportional to air gap, therefore high accuracy is not achievable.
Another system uses a time-based integrator, such as an RC circuit, to set the threshold at the average value of magnetic bias. While this system can yield high accuracy, the accuracy is not achieved until considerable amount of target rotation has taken place. It is more desirable to achieve the adaptive threshold point very quickly in the target rotation. Such is especially true in automotive applications where federally-mandated emission requirements are ever reducing allowable exhaust gases and allowable open-loop control time at start-up.
Other proposals, such as that proposed by U.S. Pat. No. 5,650,719, include digital schemes for tracking high and low voltage peaks along with voltage minimums of the output waveforms. After tracking, the schemes are setup to select a point between the high and low peaks for the adaptive threshold, and thereafter update these peak and minimum values on a regular basis determined by a selected passage of target features.
However, all the known schemes for setting a threshold to compensate for the sensor inaccuracies to minimize switch point deviation suffer drawbacks. Such drawbacks may include increased circuit complexity, which leads to increased expense; extensive target rotation before the adaptive threshold is determined; and lessened overall accuracy of the determined adaptive threshold for the waveform variance.
Compromises among these negatives are inherent in any design. The present invention, however, seeks to minimize the deleterious tradeoffs and provide a magnetic sensor that provides an adequate balance of low cost, fast threshold acquisition time, and high accuracy.